That invaluable rule of our Lord's, "To do to others as we would have them do to us," will serve to explain, when rightly understood, the true character of moral instruction. If you were to understand that precept as designed to convey to us the first notions of right and wrong, and to be your sole guide as to what you ought to do and to avoid in your dealings with your neighbor, you would be greatly perplexed. For you would find that a literal compliance with the precept would be some times absurd, sometimes wrong., and sometimes impossible. And probably it is through making this mistake that men in general apply the rule so much seldomer than they ought. For the real occasions for its use occur to all of us every day.
Richard Whately, Lessons on Morals and Christian Evidences (PDF), Lesson IV. Whately goes on to note that application of the Golden Rule presupposes both that you have a conscience and are reasonable and suggests that the "real design of it is to put us on our guard against the danger of being blinded by self-interest."